1. Field of the Invention
This invention relates to a hologram scale for measuring relative displacement quantities between two relatively displaceable mechanical components. More specifically, it relates to a hologram scale mounted on a linear encoder having high resolution.
2. Description of the Prior Art
Optical scales for measuring relative displacement quantities between two mechanical components have been generally known as metallic pattern scales which are made by the method of applying photoresists on the surface of a metallic substrate glass scale, then projecting patterns having desired pitches by a continuous step and repeat method, and after projection, supplying a development treatment or etching for the glass scale. These metallic scales have pitches on an order of 10 microns.
Recently however, processing accuracy on the order of 0.1 microns has been required for some applications, such as when processing spindles for video tape recorders. Therefore, conventional metallic scales are not sufficient as a displacement measuring system.
In order to solve the above problem, holographic techniques have been applied in this field. A typical process for producing a hologram scale is shown in FIG. 6. The surface of a base substrate 1, essentially a glass substrate, is adhered to a hologram film 2 formed by an emulsion of silver salts, dichromated gelatin (DCG) or the like. Laser beams L.sub.1 and L.sub.2 which are parallelized laser beams having specified wavelength .lambda. are irradiated so as to intersect at a desired angle which causes interference fringes to be formed at regular pitches P. These pitches, caused by the intersection of lasers L.sub.1 and L.sub.2 are transferred to the hologram film 2 directly. Then, if silver salts are used for emulsion of the hologram film 2, development and bleaching are performed so that the interference fringes are fixed, forming a scale with the pitches P as its unit of measure (a kind of diffraction grating). This diffraction grating may be considered as phase type hologram or volume type hologram. The pitches P of the hologram scale can be practically formed at intervals of 0.5 microns, so a resolution of twenty times that of conventional metallic pattern scales can be achieved.
To further improve processing accuracy, linear encoders having high resolutions are desired. Hologram scales as aforementioned are utilized for such linear encoders, which have strict accuracy and resolution requirements.
However, generally, holograms formed on such as a glass scale are strongly affected by humidity. For example, when a glass scale is exposed to temperatures of 30.degree. C. or so, and a humidity of about 60% RH for extended periods, the scale, depending on differences in the refractive index of a hologram, becomes unreadable and cannot be used as a scale. Therefore, sealed holograms have been required to increase moisture resistance.
Japanese Patent First Publication (Tokkai) No. 62-32485 discloses a hologram scale as shown in FIGS. 8 and 9. A surface of a glass base substrate 1, with a hologram film 2 adhered, is fixed to a cover substrate 4, which widely surrounds the hologram, for protection and fixed to the glass base substrate by light transmitting adhesives 3. The width of the base substrate 1 is generally narrower than that of the cover substrate 4, and this difference in width is made up by an adhesive composed of epoxy resins (EP) which substantially surround the base substrate 1 and effectively bonds it to the cover substrate 4. According to this prior art, as shown in FIG. 9, the hologram film 2 can substantially be sealed from ambient air by adhesives 5, so that the moisture resistance of the hologram film 2 can be improved. However, in this type of hologram scale, the linear expansion coefficients of the adhesive 5, the cover substrate 4 and the base substrate 1 are different so that curvature of the scale per se is caused when the scale faces high temperatures, and peeling of the base substrate 1 is caused when the scale faces low temperatures. Additionally, the aforementioned type of the scale is of complex structure and maufacture of such a scale takes a long time because the process for forming this comprises two steps using two adhesives 3, and 5.
Japanese Patent First Publication (Tokkai) No. 57-146283 discloses a sealing means for a hologram as shown in FIG. 10. In FIG. 10, a hologram film 22 is adhered to a glass substrate 23 and a protective glass substrate 21 is fitted therebelow via spacers 24. The outer sides of the spacers 24 are sealed by adhesives 25. A cavity 26 is defined by the hologram film 22 and the protective glass substrate 21 which is sealed facing each other by adhesives 25, and in this cavity 26, dry air is trapped. According to this structure, deterioration of the hologram can be prevented because the hologram film 22 is exposed to an environment having relatively low humidity. Alternatively, a modified embodiment of the seal means for the hologram of FIG. 10 is shown in FIG. 11. In this case, parts of a hologram film 22a and 22b are removed, then the ends of the spacers 24 come in direct contact with the glass substrate 23. According to this modification, the adhesive strength of the adhesives 25 can be increased because, since hologram films are often formed of gelatin etc. and do not provide a strong bond with adhesives, the spacers 24 and adhesives 25 do not directly contact the hologram film 2.
Another sealing means for a hologram is disclosed in Japanese Patent First Publication (Tokkai) No. 61-6681 as shown in FIG. 12. Referring now to FIG. 12 whose reference numerals are similar to FIG. 10, on a surface of a hologram film 22 adhered to a glass substrate 23, a protective glass substrate 21 is adhered through a first resin layer 27 and a second resin layer 28. The first and second resin layers 27, 28 are peripherally sealed with a sealing material 29. According to this disclosure, deterioration of the hologram can be prevented because the hologram film 22 is sealed from ambient environmental conditions by the first and second resin layers 28 and the seal material 29. Alternatively, deterioration of the hologram can also be prevented if the second resin layer 28 is applied only directly over the surface of the hologram film 22 as shown in FIG. 13.
However, In the previously described process in which dry air etc. is trapped in a cavity 26 within the scale, manufacture of the hologram scale is complicated and expensive. Additionally, the refractive index of the cavity 26 is greatly different from that of the protective glass substrate 23, furthermore, maintaining a uniform thickness of the cavity 26 is very difficult and so the diffraction efficiency of the hologram film 22 is compromised and becomes nonuniform, thus the accuracy of the hologram as a linear encoder tends to deteriorate.
On the other hand, in the case of coating a hologram film 22 with resin layers 27 and 28, as described above, the dispersion of the resins is difficult to maintain at a constant thickness, therefore the diffraction efficiency of the hologram film 22 is compromised and becomes unsuitable as a linear encoder. Furthermore, bubbles tend to contaminate the resins 27, 28, therefore, diffracted light from the hologram film 22 causes high turbulence, so accurate position signals cannot be obtained when the hologram is assembled as a linear encoder.
Furthermore, in the case of using a seal material 29 as a molding around the periphery of a glass substrate 23 on which a hologram film 22 is adhered with a protective glass substrate 21 further adhered thereon through relatively thick resins, the thermal expansion coefficients among the resins, the protective glass substrate 21 and the glass substrate 23 are quite different. Therefore, according to alterations in ambient temperature, curvature is caused in the seal of hologram assembly, and moreover, peeling of the interface portions of the hologram film 22 is caused by strain to the glass substrate 23 due to these conditions.